Question

Let us say I have:

// This is all valid in C++11.
struct Foo {
    int i = 42;
    int& j = i;
};

// Let's take a pointer to the member "j".
auto b = &Foo::j; // Compiler is not happy here
// Note that if I tried to get a pointer to member "i", it would work, as expected.
Foo f;
std::cout << f.*b; // Try using the pointer to member

The compiler complains that I cannot take the address of the member because it is a reference. To be precise:

Semantic Issue: Cannot form a pointer-to-member to member ‘j’ of reference type ‘int &’

I know doing this seems pointless, but I am only wondering why it cannot be done.

Why is this impossible?

Answer 1

Member pointer (as opposed to a simple pointer to a member) is simply an offset into the structure, not a pointer at all. You can get data through it only in conjunction with the structure itself (or a pointer to a structure): the value of the offset is added to the address of the structure, and the result is dereferenced to produce the value of the member.

Now suppose a member is a reference, so accessing data through it already requires a dereference (compiler hides it from us, but it needs to spit out the corresponding instructions in its output). If C++ were to allow member pointers to references, they’d be of yet another type: an offset that needs to be added to the base, and then dereferenced twice. It is too much work to improve an already obscure feature; prohibiting it is a much better way out.